Electric discharge lamps, such as fluorescent lamps, operate by applying an electric current through a gas such that at least some of the gas atoms become ionized. When enough atoms are ionized, the gas becomes an electric conductor and light radiation results.
Several circuits have been devised for starting and operating fluorescent lamps with the intent of conserving energy while maintaining correct lamp operation. The most successful methods thus far incorporate high frequency (20 KH to 30 KH) lamp excitation. Examples include U.S. Pat. No. 4,477,748 to Grubbs; U.S. Pat. No. 4,398,128 to Wollank; U.S. Pat. No. 4,251,752 to Stolz; U.S. Pat. No. 4,055,335 to Perper; U.S. Pat. No. 4,109,307 to Knoll; U.S. Pat. No. 4,329,627 to Holmes; U.S. Pat. No. 4,220,896 to Paice; U.S. Pat. Nos. 3,648,196 and 3,753,071 to Engel et al.; U.S. Pat. No. 3,890,537 to Park et al.; U.S. Pat. No. 3,710,177 to Ward; U.S. Pat. No. 3,701,925 to Nozawa et al.; U.S. Pat. No. 3,573,544 to Zonis and others.
Dimming the lamps by reducing the frequency of excitation within a high frequency circuit has been presented as another means of reducing the power requirements. Examples include U.S. Pat. Nos. 4,207,497 and 4,210,846 to Capewell et al.; U.S. Pat. No. 3,936,696 to Gray; U.S. Pat. No. 3,422,309 to Spira; and U.S. Pat. No, 3,514,668 to Johnson et al. High frequency excitation does reduce the amount of power consumed (approximately 11% to 25% depending upon the design used). High frequency designs, however, have not been well received by the industry or used in large quantities because of the high failure rates encountered. As an example, one recent high frequency design produced in quantities of several thousand yielded a failure rate so severe that the product was taken off the market by the manufacturer.
Further, all high frequency designs suffer from one or more of the following problems; they can be damaged by transient voltages from the incoming AC line; they generate R.F.I. (Radio Frequency Interference); they shorten lamp life by causing premature failure of the filaments inside many lamps; they produce frequency variations due to heating of the active power components used (SCR's, Triacs, transistors or FET's); they require many more components thereby increasing costs of production; and operation can vary from one unit to another due to sensitivity to variations in tolerances of the components used.
Because of the above problems, low frequency, core and coil ballast units have prevailed. Nevertheless, when the high frequency ballast were introduced, attention was focused on energy conservation within the lighting industry. The increased interest resulted in low frequency ballast designs that incorporated various means of reducing the amount of power consumed. This combination of events led to strong differences of opinion in the industry. On the one hand, the high frequency designs reduced operating cost as much as 25%, but they were not reliable enough to use in large quantities. On the other hand, the low frequency designs were reliable, but offered little energy savings. In the great majority of cases, minimizing the cost of production prevailed, with the low frequency core and coil designs being less expensive than the high frequency ballast.
Presently, the core and coil ballast account for approximately 98% of all ballast sales. In order to maintain this market share, the manufacturers of low frequency core and coil ballast have devised means of making their products more energy efficient. In almost every case, this invites turning off the heater current to the filaments of the lamp after the lamp has ignited.
Examples of this method are found in U.S. Pat. No. 4,399,391 to Hammer et al. using a SIDAC connected in a series circuit with the primary of the filament transformer and a capacitor. In Hammer's design, the voltage differential needed to make the SIDAC perform its switching function is derived directly from one of the lamp filaments and phase shifted through a capacitor. This method of switching could very easily result in unstable operation (lamp oscillations) due to the current differential realized through lamps connected in series. The problem would become more significant when different wattage lamps are used, since the current characteristics of 40 watt lamps are much different from 34 watt lamps.
U.S. Pat. No. 4,010,399 to Bessone et al. discloses a method of turning off the heater current (filament current) using independent circuits consisting of a Triac connected in parallel with a resistor divider. The Triac/resistor networks are then connected in series with each lamp filament (2 networks per lamp). Thermal switches have also been used to open the filament circuit inside the lamp after reaching a specified temperature. Examples of this method are found in U.S. Pat. No. 2,354,421 to Pennybacker; U.S. Pat. No. 2,462,335 to Reinhardt; and U.S. Pat. No. 4,097,779 to Latassa. The same method was also used within a ballast by locating a thermal switch next to the transformer core in U.S. Pat. No. 2,317,602 to Hall. A relay with two sets of contacts was used by Bessone in U.S. Pat. No. 4,146,820 and a magnetizable core (forming a relay type switch) was used by Raney in U.S. Pat. No. 2,330,312. magnetic reed switches were used by Latassa in U.S. Pat. No. 4,009,412 that were energized by the magnetic field generated around the transformer core. This method required that the reed switches be oriented in a specific direction and located, within critical tolerances, in that portion of the magnetic field with the highest gauss levels. In addition, the problems were compounded due to variations from one reed switch to another. As a result, this method was never used in high volume production. A less effective method was used by Sammis in U.S. Pat. No. 3,525,901 whereby the heater voltage was controlled rather than being turned completely off.
Turning heater current off does conserve energy during normal lamp operation, but the amount of energy saved is limited to the amount of current required to operate the filaments; usually 8% to 10%.
Several methods of starting fluorescent lamps have been tried to yield a means of reducing the amount of energy used during the start process. For example, U.S. Pat. No. 3,982,153 to Burdick et al. used a surge current to achieve a rapid warm-up of the heaters. U.S. Pat. No. 3,582,709 to Furui used an unignited lamp as a ballast component in the circuit. U.S. Pat. No. 4,145,638 to Kaneda used series start circuits that operated sequentially causing one lamp to ignite before the other. U.S. Pat. No. 2,697,801 to Hamilton used a thermal switch to operate a relay which controlled the amount of current going to the heaters. U.S. Pat. No. 3,866,088 to Kaneda used a backswing voltage generated by an oscillator. U.S. Pat. No. 3,720,861 to Kahanic used a time delay circuit comprised of a SCR that generated a transient spike voltage to the heaters. U.S. Pat. No. 3,588,592 to Brandstadter used a SCR to control the voltage to the heaters. U.S. Pat. No. 3,851,209 to Murakami et al. used a pulse generating circuit consisting of a pulse transformer and bi-directional diodes. U.S. Pat. No. 2,668,259 to Stutsman used gas discharge tubes within the circuit to start the fluorescent lamps. U.S. Pat. No. 4,053,813 to Kornrumpf used a transistorized inverter circuit to control the voltage by controlling the frequency of the applied power.
Presuming, however, that a fluorescent fixture will be switched on and off five times a day, that the start process takes three seconds from beginning to end, and that a normal day of operation is eight hours. The total amount of time that the start circuit is in operation over a one year period would then be 1.52 hours, or 0.05% of the total lamp operation time. When the amount of energy saved due to improved starting circuits is compared to the amount of energy consumed to operate the lamps, it seems apparent that improved starting methods contribute very little to the over-all energy efficiency of fluorescent ballast operation.
Other proposed methods of controlling the operation of fluorescent lamps will also affect the amount of energy used. U.S. Pat. No. 3,753,040 to Quenelle describes a strobing circuit using a Triac as the means of control. U.S. Pat. No. 3,449,629 to Wigert et al. uses a variable frequency oscillator circuit that can be controlled externally by heat or light sensors. Another example is found in U.S. Pat. No. 3,317,789 to Nuckolls which stabilizes lamp operation in response to variations in either heat or light. U.S. Pat. No. 3,611,021 to Wallace uses a feedback signal to reference comparator to achieve stabilization. Reversing the flow of current through fluorescent lamps have been thought to balance the light output. Examples of this method of control are found in U.S. Pat. No. 2,810,862 to Smith using a relay and U.S. Pat. No. 3,904,922 to Webb et al. using a SCR bridge circuit. The amount of flickering encountered with fluorescent lamps is controlled with parallel connected capacitors in U.S. Pat. No. 2,487,092 to Bird, while U.S. Pat. No. 2,588,858 to Lehmann solves the problem by connecting the lamps in phase relationships through multiple series connections.
With the exception of external control options, the stabilization of light output has been greatly improved through the use of more efficient coatings on the inside surfaces of the lamps. Many of the problems discussed above have now been completely eliminated through improved lamp technology.
As a result of changing markets within the lighting industry, two independent efforts are now in process. The manufacturers of high frequency ballast are directing their efforts toward reducing the price of their products while improving reliability and performance, and the manufacturers of the low frequency ballast are seeking to improve the energy efficiency of their products without increasing price. A need clearly exists for a ballast unit that offers the price and reliability of the low frequency units along with the energy efficiencies of the high frequency units.